Magnetic recording media have widely been used as audio tapes, video tapes as floppy disks. A magnetic recording medium is basically comprised of a non-magnetic support laminated with a magnetic layer containing ferromagnetic particles dispersed in a binder.
It is necessary for a magnetic recording medium to have high level characteristics such as excellent electromagnetic conversion characteristics, good running durability and good running efficiencies. That is, a high capability for reproducing original sounds is necessary for an audio tape for recording and reproducing music. Also, excellent electromagnetic conversion characteristics, such as a high capability for reproducing original images, are necessary for video tapes.
As described above, both excellent electromagnetic conversion characteristics and good running durability are required for a magnetic recording medium. Generally, abrasive agents and lubricating agents play important roles in achieving good running durability.
In order to obtain excellent running durability by adding abrasive agents, it is necessary to increase additive amounts thereof, thereby decreasing the amount of ferromagnetic particles. When abrasive agents having a large particle size are used to obtain excellent running durability, the abrasive agents are likely to extrude excessively on the surface of the magnetic layer. Accordingly, the improvement of running durability by adding abrasive agents tends to deteriorate electromagnetic conversion characteristics.
Also, when running durability is improved by adding lubricating agents, it is necessary to increase the additive amount thereof. Therefore, the binder is likely to be plasticized and thus the durability of the magnetic layer tends to decrease.
As another approach for improving running durability, the hardness of the magnetic layer has been increased by using a hard binder. However, increasing the hardness of the magnetic layer undesirably causes the magnetic layer to become strikingly fragile; drop out readily occurs when the magnetic head causes in contact with the magnetic layer and still life characteristics are deteriorated.
JP-A-58-153224 discloses a magnetic recording medium comprised of polyisocyanate, polyurethane resins having a hydroxyl group and having a molecular weight of less than 10,000 and resins which have at least one active hydrogen atom. Also, JP-A-59-136365 and U.S. Pat. No. 4,576,726 disclose a magnetic recording medium comprised of polyisocyanate and polyurethane resins in which hydroxyl groups are formed by ring-opening of expoxy rings. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".)
As described above, polyurethane resins (or other resins) having an active hydrogen (such as a hydroxyl group) are highly reactive with polyisocyanate, and therefore such polyurethane resins and polyisocyanate react sufficiently, i.e. they cross-link. Such cross-linking make the magnetic layer remarkably strong. As a result, even after a long term storage, a magnetic layer having greater output and excellent durability can be obtained. However, the above-described magnetic layer does not completely adhere to a non-magnetic support, and therefore sufficiently excellent running durability cannot always be obtained.
Accordingly, a magnetic recording medium having reduced occurrences of drop outs and excellent still-life characteristics, i.e. having excellent running durability has not yet been obtained.